The application of diamond to windows in a laser is disclosed in U.S. Pat. No. 3,895,313. The use of very pure single crystal diamonds having a thermal conductivity of at least 10 w/cm.degree.K. at 300.degree. K. makes possible the transmission of more powerful laser beams than had been achieved with other known window materials, according to this patent. The windows mentioned therein can be flat or lense shaped, single crystal or mosaic (composed of a plurality of single crystals held together by a metal grid). Other applications mentioned in this patent are in microwave devices, reflectors, and interference filters.
A compact is a polycrystalline mass of abrasive particles (e.g., diamond and cubic boron nitride) bonded together to form an integral, tough, coherent, high-strength mass. Representative U.S. Pat. Nos. on the subject of diamond compacts are: 3,136,615 (boron carbide bonding medium); 3,141,746; 3,239,321 (graphite-free diamond compact); 3,744,982 (boron alloyed diamond compact process); 3,816,085; and 3,913,280. A composite compact is a compact bonded to a substrate material, such as cemented tungsten carbide (see U.S. Pat. No. 3,745,623). Representative U.S. Pat. Nos. on the subject of cubic boron nitride (CBN) compacts are: 3,233,988; 3,743,489 (aluminum alloy catalyst); 3,767,371 (composite) and 3,852,078 (uniform compacts of polycrystalline CBN with other hard materials (e.g., diamond). U.S. Pat. Nos. 3,831,428; 4,129,052 and 4,144,739 disclose wire drawing dies made from diamond or CBN. Cutting tools made with compacts are disclosed in U.S. Pat. No. 3,850,053.
The conversion of shock-formed wurtzitic boron nitride (WBN) to CBN compacts by high pressure-high temperature (HP/HT) processing is disclosed in U.S. Pat. No. 3,876,751; German Offenlegungschrift No. 2 235 240 and U.S. patent application Ser. No. 674,430 filed Apr. 7, 1976 now abandoned, which is incorporated by reference.
A process for directly converting pyrolytic boron nitride (PBN) to polycrystalline CBN in the absence of any catalyst such as those specified in U.S. Pat. No. 2,947,617 is described in U.S. Pat. No. 4,188,194. See also--F. R. Corrigan and F. P. Bundy, "Direct Transitions Among the Allotropic Forms of Boron Nitride at High Pressure and Temperatures", The Journal of Chemical Physics, Vol. 63, No. 9 Nov. 1, 1975).
A study of the erosion resistance of single crystal and polycrystalline diamond was presented at a conference given July 25-27, 1977, at the University of Colorado; Boulder, Colo., which is published in High-Pressure Science and Technology, Vol. 2, pp. 549-558, Ed. Timmerhouse, KD and Barber, M.S., Plenum Press, New York. This paper mentions the optical window potential of diamond due to its broad transmittance band.
U.S. Pat. No. 3,829,544 mentions the possible use of white diamond compacts made without catalysts as heat sinks, solid state devices and optical devices.
U.S. Pat. No. 3,949,062, teaches the formation of polycrystalline diamond aggregates of predetermined shape.
The disadvantages of windows made from single crystal diamonds, as taught by U.S. Pat. No. 3,895,313, are the tendency of single crystals (both diamond and hard phase boron nitride) to shatter under impact loading and their scarcity and prohibitive cost in large sizes. The concept of using polycrystalline diamond or hard phase boron nitride is aimed at overcoming these disadvantages. Compacts (larger than most single crystal diamonds) are made from small crystal grains of varying sizes. Because compacts have grain boundaries, crack propagation is minimized in comparison to natural diamond which has weak planes of cleavage.
Although diamond compacts do not possess the maximum intrinsic hardness of single crystal diamond, because they are of less than full diamond density, they provide abrasion resistance equal to that of natural diamond. In hostile environments, at high velocities, abrasion resistance is an important factor. Optical windows in such environments must be able to withstand the abrasion of ice particles and water droplets at several times the speed of sound. Although such particle impacts individually last for microsecond time periods, they create repetitive high local stresses which can be severe.